1. Field of the Invention
This invention relates to an X-ray mask structure to be used for X-ray lithography, a process for preparing the same and an X-ray exposure method by use of said X-ray mask structure.
2. Related Background Art
Various methods have been used in the prior art as the lithographic working method of electronic devices such as IC, LSI, etc., but among them, X-ray lithography is based on the property of high transmittance, short wavelength, etc. inherent in X-ray and has many excellent points as compared with the lithographic methods with visible light or UV-ray hitherto practiced, thus attracting attention as a powerful means for the submicron lithographic method.
The X-ray mask structure to be used in these X-ray lithographies, for example, as shown in FIGS. 1A and 1B, comprises an X-ray transmissive film 2 supported by spanning on a supporting frame 1, and further an X-ray absorptive member 3 subjected to patterning held on said X-ray transmissive film 2.
In the preparation steps of the X-ray mask structure as shown in FIGS. 1A and 1B, above all the patterning method of the X-ray absorptive member 3 having been practiced in the prior art is a method in which the X-ray absorptive material is deposited onto the X-ray transmissive film 2 according to the sputtering method or the electrolytic plating method, and patterning is carried out by using this in combination with the resist process and the electron beam drawing method.
However, when deposition of the X-ray absorptive material is practiced according to the sputtering method or the electrolytic plating method in the patterning of the X-ray absorptive member as in the above-mentioned prior art example, the following problems have ensued.
First, in patterning of the X-ray absorptive member by use of the sputtering method, there have been involved the following problems:
(1) Since the sputtering method is a physical deposition method based on flying of the particles sputtered from a target through vacuum, no selective deposition (selective growth) of an X-ray absorptive material can be effected on the X-ray transmissive film, and therefore during patterning, two-phase etching treatments of the resist material and the X-ray absorptive material by change-over of the processing gas are required. Therefore, it has been very difficult to effect highly precise patterning of the X-ray absorptive member and control of the shape of stepped portion while suppressing the transfer error of the resist pattern onto the X-ray absorptive member pattern. PA1 (2) In the sputtering method, for controlling the grain boundary size of the grains constituting the X-ray absorptive member, the only factor is the substrate temperature control during sputtering, and hence variance will become greater in performing bulk production. PA1 (3) If the grain boundary size of the material constituting the X-ray absorptive member is desired to be made larger in the sputtering method, the internal stress of the X-ray absorptive member becomes greater, whereby warping will occur after back etching (support frame formation) of the substrate. PA1 (4) Since the electrolytic plating method is essentially a wet treatment, defects of the X-ray absorptive member pattern have been frequently generated because of fine foreign matters in the electrolytic plating solution. PA1 (5) The electrolytic plating method is liable to generate pinholes, convexities on the electrode film (undercoating film), and therefore the electrical field distribution becomes nonuniform during deposition of the X-ray absorptive material, whereby film thickness irregularity, density irregularity of the X-ray absorptive member will give rise to non-uniform distribution in X-ray absorptivity.
On the other hand, in patterning of the X-ray absorptive member by use of the electrolytic plating method, there have been involved the following problems.
Further, the X-ray absorptive member of the prior art is greatly lowered in its density as compared with the bulk density of the material constituting the X-ray absorptive member, whereby the X-ray absorptivity is lowered. Moreover, partly because the edge portion (side surface) of the X-ray absorptive member is not sufficiently smooth, there have ensued undesirable problems such as lowering in resolution, contrast, etc. during printing as the X-ray mask structure to be used for submicron lithography.